Non-contact laser-induced protrusion measurement apparatus and method

ABSTRACT

A method and apparatus are directed to providing relative movement between a slider configured for heat-assisted magnetic recording and a magnetic recording medium, and causing protrusion of a portion of an air bearing surface (ABS) of the slider in response to activating at least a laser source while maintaining spacing between the protrusion and the medium. A magnitude of at least a portion of the protrusion is measured while maintaining spacing between the protrusion and the medium.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. Ser. No. 14/964,870, filedDec. 10, 2015, which is hereby incorporated herein by reference in itsentirety.

SUMMARY

Embodiments of the disclosure are directed to a method comprisingproviding relative movement between a slider configured forheat-assisted magnetic recording and a magnetic recording medium, andcausing protrusion of a portion of an air bearing surface (ABS) of theslider in response to activating at least a laser source whilemaintaining spacing between the protrusion and the medium. The methodcomprises measuring a magnitude of at least a portion of the protrusionwhile maintaining spacing between the protrusion and the medium.

Some embodiments are directed to a method comprising providing relativemovement between a magnetic recording medium and a slider configured forheat-assisted magnetic recording, the slider comprising an air bearingsurface (ABS) and a thermal sensor at or near the ABS. The methodcomprises, in the absence of laser excitation, determining a firstresistance response of the thermal sensor to varying clearance whilemaintaining spacing between the slider and the medium. The method alsocomprises, in the presence of laser excitation, determining a secondresistance response of the thermal sensor to varying clearance whilemaintaining spacing between the slider and the medium, wherein the laserexcitation causes protrusion of a portion of the ABS. The method furthercomprises measuring a magnitude of at least a portion of the protrusionusing the first and second resistance responses.

Other embodiments are directed to an apparatus comprising a sliderconfigured for heat-assisted magnetic recording, with one or morethermal sensors at or near an air bearing surface (ABS) of the slider.Excitation of a laser source causes protrusion of a portion of the ABSextending toward, but spaced apart from, a magnetic recording medium. Aprocessor is coupled to the one or more thermal sensors and configuredto measure, while maintaining spacing between the protrusion and themedium, a magnitude of at least a portion of the protrusion using theone or more thermal sensors.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a side view of a slider configured for heat-assisted magneticrecording (HAMR) with which embodiments of the present disclosure can beimplemented;

FIG. 2 is a perspective view of a slider configured for heat-assistedmagnetic recording (HAMR) in accordance with various embodiments;

FIGS. 3A-3C are simplified side views of a writer portion of the sliderillustrated in FIGS. 1 and 2;

FIG. 4 is an exaggerated illustration of a laser-induced protrusiondeveloped at an air bearing surface (ABS) of a HAMR slider in accordancewith various embodiments;

FIG. 5 is a block diagram of an apparatus configured for measuringlaser-induced protrusion of a slider ABS using a non-contact-basedtechnique in accordance with various embodiments

FIG. 6 is a flow diagram of a non-contact-based laser-induced protrusionmeasurement in accordance with an embodiment of the disclosure;

FIG. 7 is a flow diagram of a non-contact-based laser-induced protrusionmeasurement in accordance with some embodiments;

FIG. 8 illustrates a heat sinking phenomenon at a head-disk interfaceinvolving a thermal sensor and a magnetic recording medium in accordancewith various embodiments;

FIG. 9 illustrates a scenario similar to that depicted in FIG. 8, andadditionally shows a change in a zero-bias thermal sensor resistance,R₀, due to changes in external heating due to varying slider clearancein accordance with various embodiments;

FIG. 10 illustrates how the value of a zero-bias sensor resistance, R₀,can be calculated for a thermal sensor in accordance with variousembodiments;

FIG. 11 shows a representative R₀ clearance slope (Ω/nm) derived fromexperimental R₀ data acquired with a laser off condition in accordancewith various embodiments; and

FIG. 12 illustrates a non-contact-based laser-induced protrusionmeasurement technique in accordance with various embodiments.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part of the description hereof and in which areshown by way of illustration several specific embodiments. It is to beunderstood that other embodiments are contemplated and may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description, therefore, is not to be taken in alimiting sense.

Referring now to FIG. 1, a block diagram shows a side view of a slider102 according to a representative embodiment. The slider 102 may be usedin a magnetic data storage device, e.g., a hard disk drive. The slider102 may also be referred to as a recording head or a read/write head,etc. The slider 102 is coupled to an arm 104 by way of a suspension 106that allows some relative motion between the slider 102 and arm 104. Theslider 102 includes read/write transducers 108 at a trailing edge thatare held proximate to a surface 110 of a magnetic recording medium 111,e.g., magnetic disk. The slider 102 shown in FIG. 1 is configured as aHAMR recording head, which includes a laser 120 (or other energy source)and a waveguide 122. The waveguide 122 delivers light from the laser 120to components (e.g., a near-field transducer) near the read/writetransducers 108.

When the slider 102 is located over a surface 110 of a recording medium111, a flying height 112 is maintained between the slider 102 and thesurface 110 by a downward force of arm 104. This downward force iscounterbalanced by an air cushion that exists between the surface 110and an air bearing surface 103 (also referred to as a “media-facingsurface”) of the slider 102 when the recording medium 111 is rotating.It is desirable to maintain a predetermined slider flying height 112over a range of disk rotational speeds during both reading and writingoperations to ensure consistent performance.

Region 114 is a “close point” of the slider 102, which is generallyunderstood to be the closest spacing between the read/write transducers108 and the magnetic recording medium 111, and generally defines thehead-to-medium spacing 113. To account for both static and dynamicvariations that may affect slider flying height 112, the slider 102 maybe configured such that a region 114 of the slider 102 can beconfigurably adjusted during operation in order to finely adjust thehead-to-medium spacing 113. This is shown in FIG. 1 by a dotted linethat represents a change in geometry of the region 114. In this example,the geometry change may be induced, in whole or in part, by an increaseor decrease in temperature of the region 114 via a heater 116. A thermalsensor 115 is shown situated at or near the close point 114 (e.g.,adjacent the read/write transducers 108, such as near the near-fieldtransducer) or can be positioned at other locations of the ABS 103 whereprotrusion of the ABS 103 is to be measured.

FIG. 2 shows a recording head arrangement 200 in accordance with variousembodiments. More particularly, the recording head arrangement 200 isconfigured as a HAMR device. The recording head arrangement 200 includesa slider 202 positioned proximate a rotating magnetic medium 211. Theslider 202 includes a reader 204 and a writer 206 proximate the ABS 215for respectively reading and writing data from/to the magnetic medium211. The writer 206 is located adjacent a near-field transducer (NFT)210 which is optically coupled to a light source 220 (e.g., laser diode)via a waveguide 222. The light source 220 can be mounted external, orintegral, to the slider 202. The light source 220 energizes the NFT 210via the waveguide 222. The writer 206 includes a corresponding heater207, and the reader 204 includes a corresponding heater 205 according tovarious embodiments. The writer heater 207 can be powered to causeprotrusion of the ABS 215 predominately in the ABS region at orproximate the writer 206, and the reader heater 205 can be powered tocause protrusion of the ABS 215 predominately in the ABS region at orproximate the reader 204. Power can be controllably deliveredindependently to the heaters 207 and 205 to adjust the fly height (e.g.,clearance) of the slider 202 relative to the surface of the recordingmedium 211.

FIG. 2 further shows a thermal sensor 212 situated at various locationson the slider 202 at or near the ABS 215. In general, one or morethermal sensors 212 can be situated at locations of the slider 202 wherea protrusion of the ABS 214 is to be measured. In some embodiments, athermal sensor 212 a can be situated adjacent the NFT 210 (e.g., betweenthe NFT 210 and the write pole of the writer 206). In some embodiments,the thermal sensor 212 a can be located between about 2 and 5 μm fromthe NFT 210 (or the laser focus location), such as between about 2 and 3μm. In other embodiments, the thermal sensor 212 a can be located asmuch as about 10 μm from the NFT 210 or the laser focus location.

In other embodiments, a thermal sensor 212 b can be situated adjacentthe write pole of the writer 206 on the side opposite that nearest theNFT 210. In further embodiments, a thermal sensor 212 c can be situatedadjacent the waveguide 222 on the side opposite that nearest the NFT210. Yet in other embodiments, a thermal sensor 212 d can be situatedadjacent the reader 204. It is understood that a single or multiplethermal sensors 212 may be provided/distributed on the slider 202. Thethermal sensor 212 can have a width of between about 0.5 and 10 μm, suchas about 1.5 μm. It is further understood that the thermal sensors canbe implemented in a variety of technologies, such as resistance thermalsensors, thermistors, and thermocouples, for example. Certainembodiments disclosed herein are directed to sensors having atemperature coefficient of resistance (referred to herein as TCRsensors), it being understood that other forms and/or means of sensingtemperature are considered as being within the metes and bounds of theinstant disclosure. In some embodiments, existing components of theslider 202 can be used as a thermal sensor. The reader 204 or NFT 210,for example, can be used as thermal sensors rather than or in additionto one or more dedicated thermal sensors.

Some of the TCR sensors described herein are referred to as Dual-endedThermal Coefficient of Resistance (DETCR) sensors. A DETCR sensor isconfigured to operate with each of its two electrical contacts (i.e.,ends) connected to respective bias sources provided by a pair ofelectrical bond pads of the slider 202. Another example of a TCR sensoris a ground-split (GS) temperature coefficient of resistance sensor, inwhich one end of the GSTCR is coupled to ground and the other is coupledto a bias source via an electrical bond pad of the slider 202.

A HAMR device utilizes the types of optical devices described above toheat a magnetic recording media (e.g., hard disk) in order to overcomesuperparamagnetic effects that limit the areal data density of typicalmagnetic media. When writing with a HAMR device, the electromagneticenergy (e.g., laser or light) is concentrated onto a small hotspot 213over the track of the magnetic medium 211 where writing takes place, asshown in FIG. 2. The light from the source 220 propagates to the NFT210, e.g., either directly from the source 220 or through the modeconverter or by way of a focusing element. Other optical elements, suchas couplers, mirrors, prisms, etc., may also be formed integral to theslider.

As a result of what is known as the diffraction limit, opticalcomponents cannot be used to focus light to a dimension that is lessthan about half the wavelength of the light. The lasers used in someHAMR designs produce light with wavelengths on the order of 700-1550 nm,yet the desired hot spot 213 is on the order of 50 nm or less. Thus, thedesired hot spot size is well below half the wavelength of the light.Optical focusers cannot be used to obtain the desired hot spot size,being diffraction limited at this scale. As a result, the NFT 210 isemployed to create a hotspot on the media.

The NFT 210 is a near-field optics device configured to generate localsurface plasmon resonance at a designated (e.g., design) wavelength. TheNFT 210 is generally formed from a thin film of plasmonic material(e.g., gold, silver, copper) on a substrate. In a HAMR slider 202, theNFT 210 is positioned proximate the write pole of the writer 206. TheNFT 210 is aligned with the plane of the ABS 215 parallel to theread/write surface of the magnetic medium 211. The NFT 210 achievessurface plasmon resonance in response to the incident electromagneticenergy. The plasmons generated by this resonance are emitted from theNFT 210 towards the magnetic medium 211 where they are absorbed tocreate the hotspot 213. At resonance, a high electric field surroundsthe NFT 210 due to the collective oscillations of electrons at the metalsurface (e.g., substrate) of the magnetic medium 211. At least a portionof the electric field surrounding the NFT 210 tunnels into, and getsabsorbed by, the magnetic medium 211, thereby raising the temperature ofthe spot 213 on the medium 211 as data is being recorded.

As was previously discussed, laser light produced by the laser 220 iscoupled to the NFT 210 via the waveguide 222. The NFT 210, in responseto the incident laser light, generates a high power density in anear-field region that is directed to the magnetic storage medium 211.This high power density in a near-field region of the NFT 210 causes anincrease in local temperature of the medium 211, thereby reducing thecoercivity of the magnetic material for writing or erasing informationto/at the local region of the medium 211. A portion of the laser lightenergy communicated to the NFT 210 is absorbed and converted to heatwithin the slider 215. This heating results in thermal expansion of theABS materials, protrusion at the ABS 215, and a change in bothhead-media clearance and head-media separation. In addition to the NFT210, the slider 202 typically includes additional heat sources that cancause further thermal expansion and protrusion of the ABS 215. Suchadditional heat sources, when active, include one or more of the writer206 (writer coil), writer heater 207, and reader heater 204.

FIGS. 3A-3C are simplified side views of a writer portion of the slider202 illustrated in FIGS. 1 and 2. FIGS. 3A-3C show general protrusionprogression of a portion of the slider ABS 215 in response to activationof different heat sources within the slider 202. These different heatsources include the write coil of the writer 206, the writer heater 207(not shown for simplicity), and the laser 220 (not shown for simplicity)which produces the optical energy converted to heat by the NFT 210.

In FIG. 3A, the slider 202 is shown in a non-thermally actuated state.In this state, the laser 220, writer heater 207, and writer coil 206 areall off. Thus, the slider 202 attains a default, non-actuatedshape/state establishing a default distance between the ABS 215 of theslider 202 and the surface of the magnetic storage medium 211. Thisdefault distance is illustrated by an air gap 270.

FIG. 3B illustrates the slider 202 in a partial-thermally actuatedstate, which is not a typical operational state but is shown forillustrative purposes. In this state, the writer heater 207 and thewriter coil 206 are on, but the laser 220 is off. In response toactivation of the writer heat sources (write pole, return pole) andwriter heater 207, the ABS 215 at and surrounding the writer portion ofthe slider 202 protrudes into the air gap 270. Thus, the air gap 270 andthe distance between ABS 215 and the medium surface 211 decreases. Thedashed line in FIG. 3B indicates the default state/shape of ABS 215depicted in FIG. 3A.

The magnitude of ABS protrusion of the slider 202 is furthered increasedby the additional activation of the laser 220, as shown in FIG. 3C. Theadditional heat produced by the NFT 210 in response to the incidentlaser light further expands the ABS 215, causing the ABS 215 to protrudefurther into air gap 270. It can be seen in FIGS. 3A-3C that the stroke,or magnitude, of the air bearing surface protrusion along the crosstrack direction (z-axis) of the slider 202 changes in size and shapewith introduction and removal of heat to/from the ABS 215.

FIG. 4 is an exaggerated illustration of a laser-induced protrusiondeveloped at the ABS 215 of a HAMR slider 200 in accordance with variousembodiments. More particularly, the protrusion of the slider ABS 215shown in FIG. 4 is referred to herein as Laser-induced Writer Protrusion(LIWP). As a shown in FIG. 4, the region of LIWP encompasses a writer206 and an NFT 210 of the slider. LIWP represents the full excursion ofthe protrusion developed at the ABS 215 due to heating of the NFT 210 byexcitation of the laser and other heat sources (e.g., the writer 206 andwriter heater 207). The reader 204 is also subject to displacement bythe ABS protrusion resulting from excitation of the laser of the slider.Protrusion of the slider ABS 215 due to laser/NFT heating in the regionthat encompasses the reader 204 is referred to herein as Laser-inducedReader Protrusion (LIRP). Because the reader 204 is situated away fromthe NFT 210/writer 206, allowing for dissipation of laser-induced heat,LIRP is not as pronounced as LIWP. However, LIRP is quite noticeable andimpacts reader performance. It is noted that the features shown in FIG.4 are not drawn to scale.

LIWP is understood to include two protrusion components. The firstcomponent of LIWP is a broad protrusion component, referred to herein asBroad Laser-induced Writer Protrusion (BLIWP). As the term implies, arelatively broad region of the ABS 215 surrounding the writer 206 andNFT 210 expands to form a protruded region (volume) R1 215 a in responseto the heat generated by the NFT 210 and the writer 206 (and writerheater 207). The second component of LIWP is a local protrusioncomponent, referred to herein as Local Laser-induced Writer Protrusion(LLIWP). LLIWP is a small and narrow protrusion (relative to the BLIWP)that extends from the BLIWP in a direction towards the surface of themagnetic recording medium 211. As can be seen in FIG. 4, the BLIWPcomponent encompasses a significantly larger volume (in region R1 215 a)of ABS material relative to that (in region R2 215 b) of the LLIWPcomponent. Evaluation of experimental sliders has revealed that LIWPtypically ranges between about 2 and 4 nm, while LLIWP typically rangesbetween about 1 to 2 nm (<2 nm). It is understood that, although each ofLIWP, BLIWP, LLIWP, and LIRP involves expansion of a volume of ABSmaterial, these protrusion parameters are measured in terms of adistance (in nanometers) extending from the ABS 215 and along a planenormal to the ABS 215 in a direction towards the recording medium 211.

As was discussed previously, excitation of the laser causes opticalenergy to impinge on the NFT 210, causing significant heating at the ABS215 in the region of the NFT 210. The heat produced by the NFT 210 andthe writer 206 (and other thermal sources, such as the writer heater,reader, and reader heater) causes thermal expansion of the surroundingABS material, resulting in the BLIWP. Heating of the NFT 210 alsoresults in high power density in the local region immediatelysurrounding the NFT 210, resulting in development of the LLIWP. Althoughthe ABS material in region R1 subject to BLIWP and that of region R2subject to LLIWP is essentially the same, the thermal time constant ofthe material in region R1 and region R2 vary significantly from oneanother. For example, the thermal time constant of the material inregion R1 (subject to BLIWP) is between about 100 and 200 μs, which issimilar to that of ABS material subject to heating by the writer heateror the reader heater. The thermal time constant of the material inregion R2 (subject to LLIWP) is around 1 μs or less.

An important function of a hard disk drive (HDD) is to accurately setthe clearance between the slider and the surface of the magnetic storagemedium of the HDD. Toward this end, various techniques have beendeveloped to set clearance that involve incrementally reducing flyheight of the slider until contact is made between the slider and therecording medium. Once contact is made, an appropriate clearance is setsuch that slider is made to fly close to, but spaced apart from, thesurface of the medium during operation. It can be appreciated that forHAMR sliders, it is important to account for LIWP in order to avoiddetrimental contact between the slider and the medium. Conventionaltechniques that account for LIWP when setting clearance require that theslider be forced into contact with the recording medium. Suchconventional techniques are generally regarded as destructive, in thatthe slider used for setting clearance is damaged or destroyed duringclearance testing. Although clearance settings determined using testsliders can be used for setting clearance of HDD sliders of similardesign, conventional clearance setting techniques cannot be used in situan HDD due to the destructive nature of these techniques.

Embodiments of the present disclosure are directed to techniques formeasuring laser-induced protrusions in HAMR sliders that do not requirecontact between the slider and the surface of the magnetic recordingmedium. More particularly, embodiments of the disclosure are directed totechniques for measuring a magnitude of at least a portion of alaser-induced protrusion in a HAMR slider while maintaining spacingbetween the protrusion and the surface of the magnetic recording medium.Embodiments are directed to measuring the magnitude of a broad region ofa laser-induced protrusion of a HAMR slider without contacting therecording medium, obtaining an estimate of the magnitude of a localregion of laser-induced protrusion that extends from the broad region,and setting slider clearance using the broad region measurement and thelocal region estimate.

According to some embodiments, changes in the resistance of a thermalsensor of the slider are used to measure one or both of BLIWP and LIRP.The thermal sensor can be used for measuring BLIWP or LIRP duringreal-time testing, such as during component testing on a spin stand, orin situ an HDD during the lifetime of the product. As is discussedbelow, the resistance signal produced by the thermal sensor can bemeasured by several methods including sweeping AC or DC bias of thethermal sensor with the laser on and off, and sweeping one or moreheaters to control clearance. The thermal sensor resistance measurementmethod can be chosen to optimize the signal response for a given sliderdesign. The thermal sensor is preferably located at locations wherelaser-induced protrusion (e.g., BLIWP or LIRP) measurements are to bemade.

FIG. 5 is a block diagram of an apparatus configured for measuringlaser-induced protrusion of a slider ABS using a non-contact-basedtechnique in accordance with various embodiments. The apparatus 500shown in FIG. 5 can be implemented in an HDD or in a spin stand tester,for example. A slider 202 of the apparatus 500 includes a thermal sensor212 situated at a location at which laser protrusion of the ABS 215 isto be evaluated. The sensor 212 can, for example, be located near theNFT, writer, or reader, or other location at which ABS protrusion is ofinterest. The slider 202 also includes a heater 216 located in the areain which laser protrusion of the ABS 215 is to be evaluated. A laser220, such as a laser diode, is situated on the slider or proximate theslider 202, and is configured to couple light to an NFT (not shown) ofthe slider 202 via an optical waveguide. A power supply 504 is coupledto the slider 202 and is configured to bias the heater 216 and bias thethermal sensor 212. It can be appreciated that the power supply 504includes multiple power supplies for individually controlling bias powersupplied to the heater 216 and the thermal sensor 212. A processor 502is coupled to the slider 202 and is configured to coordinatenon-contact-based measuring of laser-induced protrusion of the ABS 215in accordance with various embodiments.

A clearance setting procedure according to various embodiments involvesproviding relative movement between the slider 202 and a magneticrecording medium 211. The processor 502 cooperates with the power supply504 to measure a resistance of the thermal sensor 212 at variousclearance settings with and without exciting the laser 220. A dependenceof zero bias resistance, R₀, on clearance is calculated by the processor502 at each clearance setting. The processor 202 uses the resistancedata acquired from the thermal sensor 212 to measure a magnitude of atleast a portion of the ABS protrusion 215. Importantly, the resistancemeasurements used to measure laser-induced ABS protrusion are acquiredwhile maintaining spacing between the ABS protrusion 215 and the thermalsensor 212. The non-contact-based protrusion measurement procedure canbe repeated at various locations across the diameter of the medium 211,such as one or more outer diameter (OD) locations, one or more innerdiameter (ID) locations, and one or more middle diameter (MD) locations.

The apparatus shown in FIG. 5 (and in FIGS. 1 and 2) can be used toimplement non-contact-based protrusion measurements in accordance withvarious embodiments. By way of example, and with reference to FIG. 6,there is illustrated a flow diagram of a non-contact-based laser-inducedprotrusion measurement in accordance with an embodiment of thedisclosure. The method shown in FIG. 6 involves providing 602 relativemovement between a HAMR slider and a magnetic recording medium. Themethod also involves causing 604 protrusion of a portion of the sliderABS in response to laser activation while maintaining spacing betweenthe slider and the medium. The method further involves measuring 606 amagnitude of at least a portion of the protrusion while maintainingspacing between the protrusion and the medium.

FIG. 7 illustrates a non-contact-based laser-induced protrusionmeasurement technique that can be implemented by the apparatus shown inFIGS. 1, 2, and 5 in accordance with various embodiments. The methodshown in FIG. 7 involves providing 702 relative movement between a HAMRslider and a magnetic recording medium. The slider includes a thermalsensor at or near the slider ABS in the area in which laser-inducedprotrusion is to be evaluated. With the laser off, the method involvesdetermining 704 a first resistance response of the thermal sensor tovarying clearance while maintaining spacing between the slider and themedium. With the laser on, the method involves determining 706 a secondresistance response of the thermal sensor to varying clearance whilemaintaining spacing between the slider and the medium. The method alsoinvolves measuring 708 a magnitude of at least a portion of theprotrusion using the first and second resistance responses.

FIG. 8 illustrates a heat sinking phenomenon within a head-diskinterface, such as occurs between a thermal sensor of a slider and amagnetic recording medium. It is assumed that the slider and the thermalsensor are hotter than the recording medium due to heating by the NFTand writer (and other heat sources) and due to biasing of the thermalsensor. As such, the relatively cool recording medium acts as a heatsink. At larger clearances, heat sinking between the thermal sensor andthe recording medium is relatively poor. At smaller clearances, heatsinking between the thermal sensor and the recording medium isrelatively good.

FIG. 8 shows two plots 802 and 804 of thermal sensor resistance, R (Ohm)as a function of sensor bias power (mW). Plot 802 represents a scenarioin which the thermal sensor is relatively far away from the recordingmedium (i.e., large clearance), resulting in relatively poor heatsinking between the thermal sensor and the medium. Plot 804, incontrast, represents a scenario in which the thermal sensor isrelatively close to the recording medium (i.e., small clearance),resulting in relatively good heat sinking between the thermal sensor andthe medium. As can be seen in FIG. 8, plots 802 and 804 have differentslopes. The slopes of the plots 802 and 804 represent bias sensitivityof the thermal sensor with different materials between the thermalsensor and the medium. As can be seen in FIG. 8, each of the two plots802 and 804 originate at a common point on the y-axis, designated R₀. R₀represents the value of sensor resistance, R, at zero bias power, whichis an extrapolated value. It is noted that the value of sensorresistance, R, is given by the equation R=R₀+sP_(B), where R₀ is sensorresistance at zero bias power, s is bias sensitivity (slope), and P_(B)is bias power.

FIG. 9 illustrates a scenario similar to that depicted in FIG. 8, withplot 902 indicating relatively poor heat sinking and plot 904 indicatingrelatively good heat sinking between the thermal sensor and therecording medium. FIG. 9 differs from that of FIG. 8 in that theresistance R₀ is different for each of the two plots 902 and 904. Moreparticularly, the value of R₀ for plot 904 is translated upwards on they-axis relative to R₀ for plot 902. This translation or difference inthe value of R₀ for the two plots 902 and 904 results from differencesin external heating or condition of the thermal sensor. An appreciabledifference between the value of R₀ in the two plots 902 and 904indicates that the thermal environment of the thermal sensor has changeddue to changes in slider fly height. This change in the value of R₀ dueto a change in slider clearance is used as part of a non-contactlaser-induced protrusion measurement according to various embodiments.

FIG. 10 illustrates how the value of R₀ can be calculated for a thermalsensor in accordance with various embodiments. FIG. 10 illustrates twoplots 1002 and 1004 developed from resistance, R, measurements for thesame thermal sensor at two difference clearances A and B. It isunderstood that plots for more than two clearance settings (e.g., 3-5clearance settings) can be developed, and that the two plots shown inFIG. 10 are for simplicity of explanation. Plots 1002 and 1004 can beobtained by sweeping the bias power (AC or DC) between a low value(e.g., 0 mW) and a high-value (e.g., 350 mW). As the sensor bias poweris swept, the sensor resistance, R, is measured at various power levels.This procedure is repeated with the laser off and with the laser on.

In the example shown in FIG. 10, resistance measurements are taken at 50mW intervals. A linear fit of the data is performed to calculate thebias sensitivity, s, represented by the slope of each of the two plots1002 and 1004. Using the linear fit of resistance measurements for eachplot 1002 and 1004, the value of R₀ is extrapolated for a condition ofzero bias power (with the laser off and with the laser on). The value ofR₀ at each clearance setting can be stored for future reference. Thevalue of R₀ for clearance A is smaller than that for clearance B,indicating that clearance B is larger than clearance A. After the valueof R₀ at each clearance setting has been determined, an R₀ clearanceslope is determined using the values of R₀ obtained with the laser off,as is illustrated in FIG. 11. FIG. 11 shows a representative R₀clearance slope (Ω/nm) derived from experimental R₀ data acquired withthe laser off.

FIG. 12 illustrates a non-contact-based laser-induced protrusionmeasurement technique which can be implemented by the apparatuses shownin FIGS. 1, 2, and 5 in accordance with various embodiments. The methodshown in FIG. 12 is directed to calculating the broad component oflaser-induced writer protrusion, BLIWP, for a slider ABS in accordancewith various embodiments. The non-contact BLIWP calculation method shownin FIG. 12 can begin with the laser off, and involves measuring 1202 theresistance and R₀ of one or more thermal sensors at passive and thevarious clearances. It is understood that a passive clearance representsa clearance achieved by the slider with no power being delivered to theheater situated near the ABS location to be evaluated. The resistanceresponse of the thermal sensor can be measured without the laser on at apassive clearance and at an active 3, 2, and 1 nm clearance by sweepingthe DC voltage bias on the thermal sensor within reasonable ranges(e.g., 200 to 600 mV with a dual DETCR implementation). From the thermalsensor response data, R₀ can be calculated for each clearance in amanner previously discussed. To reiterate, R₀ is the resistance of thethermal sensor with no bias applied.

From the acquired values of R₀ at the various clearances, a slope can becalculated to determine 1204 the dependence of thermal sensor resistancewith clearance for the laser off condition, such as is shown in FIG. 11.After the R₀ clearance slope for the thermal sensor has been measuredwith the laser off, the above-described process can be repeated 1206,1208 at a passive, 3, 2, and 1 nm active clearance with the laser on atoperating laser current using a conservative fixed LIWP back-off to setactive clearance, making sure that the slider never comes into contactwith the medium with a laser on. From the laser on and off thermalsensor resistance values determined at steps 1202-1208, BLIWP can becalculated 1210. According to some embodiments, clearance of the slidercan be set 1212 using the calculated value of BLIWP and an estimatedvalue of LLIWP. It is noted that an estimate of LLIWP can be obtainedempirically or through modeling, and is typically less than about 2 nm.

According to some embodiments, BLIWP can be measured (in nanometers)using the following equation:BLIWP (nm)=(R _(p) −R _(p+1))*γ/R _(0slope))where, R₀ is the zero bias resistance of the thermal sensor, R_(p) isthe passive resistance of the sensor, R_(P+1) is the passive resistanceof the sensor with current supplied to the laser, γ is a laser gammacorrection factor based on the location of the thermal sensor, andR_(0slope) is the slope of R₀ with the laser off. It is noted that thevalue of γ typically ranges between 1.0 and 2.5, where 1.0 represents nocorrection.

Experiments were performed on a number of sliders of equivalent designto measure BLIWP, data from which are presented below in Table 1. Foreach slider, the passive and active (laser on) values of R₀ weremeasured from which an R₀ difference (delta) was computed. The R₀ slopefor each slider with the laser off was computed. The gamma correctionfactor was the same for each slider due to the thermal sensor beingsituated at the same location relative to the NFT/writer for eachslider. From these data, BLIWP was calculated. For example, BLIWP foreach slider can be calculated by dividing the data of column C by thatof D, and multiplying this result by the data of column E.

TABLE 1 D G A B C R0 E Contact Sensor Sensor Sensor Slope Sensor BasedR0 (Ω) R0 (Ω) ΔR0 (Ω/nm) Gamma F BLIWP Slider Passive Laser On (Ω) LaserOff (γ) BLIWP (nm) (nm) 1 173.49 180.04 −6.55 −5.70 2.30 2.64 2.75 2174.75 180.33 −5.58 −5.43 2.30 2.37 2.42 3 172.59 179.05 −6.45 −5.082.30 2.92 2.86 4 175.15 182.97 −7.81 −5.68 2.30 3.16 2.75 5 174.22182.14 −7.92 −5.55 2.30 3.28 2.86

The values of BLIWP obtained using a non-contact-based measuringmethodology of the present disclosure are in good agreement with acontact-based technique that was laser used on the same sliders (e.g.,acoustic emission technique). It is noted that the experimental sliderswere damaged or destroyed after completing the contact-based techniqueused to corroborate the experimental results.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations can besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present disclosure. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisdisclosure be limited only by the claims and the equivalents thereof.All references cited within are herein incorporated by reference intheir entirety.

What is claimed is:
 1. A method, comprising: providing relative movementbetween a magnetic recording medium and a slider configured forheat-assisted magnetic recording, the slider comprising an air bearingsurface (ABS) and a thermal sensor at or near the ABS; causingprotrusion of a portion of the ABS of the slider in response toactivating at least a laser source while maintaining spacing between theprotrusion and the medium; in the absence of laser excitation,determining a first resistance response of the thermal sensor to varyingbias at two or more clearances while maintaining spacing between theslider and the medium; in the presence of laser excitation, determininga second resistance response of the thermal sensor to varying bias powerat the two or more clearances while maintaining spacing between theslider and the medium; extrapolating a resistance response of thethermal sensor with no bias power, R₀, using one or both of the firstresistance response and the second resistance response; and determininga magnitude of at least a portion of the protrusion using R₀.
 2. Themethod of claim 1, further comprising: determining an R₀ clearance slopeusing the first resistance response; and determining the magnitude of atleast the portion of the protrusion using the R₀ clearance slope.
 3. Themethod of claim 1, wherein: the protruded portion of the ABS comprises afirst region and a second region; the second region is closer to themedium than the first region; and determining the magnitude of at leastthe portion of the protrusion comprises determining the magnitude of thefirst region.
 4. The method of claim 1, wherein: the protruded portionof the ABS comprises a first region having a first thermal time constantand a second region having a second thermal time constant; the secondregion is closer to the medium than the first region; and determiningthe magnitude of at least the portion of the protrusion comprisesdetermining the magnitude of the first region.
 5. The method of claim 1,wherein the portion of the ABS comprises a writer and a near-fieldtransducer of the slider.
 6. The method of claim 1, wherein the portionof the ABS comprises a reader of the slider.
 7. The method of claim 1,further comprising determining the magnitude of at least the portion ofthe protrusion at different diameters or zones of the medium.
 8. Anapparatus, comprising: a slider configured for heat-assisted magneticrecording and comprising one or more thermal sensors at or near an airbearing surface (ABS) of the slider, wherein excitation of a lasersource causes protrusion of a portion of the ABS extending toward, butspaced apart from, a magnetic recording medium; and a processor coupledto the one or more thermal sensors and configured to: in the absence oflaser excitation, determine a first resistance response of the thermalsensor to varying bias power at two or more clearances while maintainingspacing between the slider and the medium; in the presence of laserexcitation, determine a second resistance response of the thermal sensorto varying bias power at the two or more clearances while maintainingspacing between the slider and the medium; extrapolate a resistanceresponse of the thermal sensor with no bias power, R₀ using one or bothof the first resistance response and the second resistance response; anddetermine a magnitude of at least a portion of the protrusion using R₀.9. The apparatus of claim 8, wherein: the protruded portion of the ABScomprises a first region and a second region; the second region iscloser to the medium than the first region; and the processor isconfigured to determine the magnitude of the first region.
 10. Theapparatus of claim 8, wherein: the protruded portion of the ABScomprises a first region having a first thermal time constant and asecond region having a second thermal time constant; the second regionis closer to the medium than the first region; and the processor isconfigured to determine the magnitude of the first region.
 11. Theapparatus of claim 8, wherein: one of the thermal sensors is situated ateach of a plurality of slider locations subject to ABS protrusion inresponse to excitation of the laser source; and the processor isconfigured to determine the magnitude of at least a portion of each ABSprotrusion using the thermal sensor proximate each ABS protrusion. 12.The apparatus of claim 8, wherein the portion of the ABS comprises awriter and a near-field transducer of the slider.
 13. The apparatus ofclaim 8, wherein the portion of the ABS comprises a reader of theslider.
 14. The apparatus of claim 8, wherein the processor isconfigured to determine the magnitude of at least the portion of theprotrusion at different diameters or zones of the medium.
 15. A method,comprising: providing relative movement between a magnetic recordingmedium and a slider configured for heat-assisted magnetic recording, theslider comprising an air bearing surface (ABS) and a thermal sensor ator near the ABS; causing protrusion of a portion of the ABS of theslider in response to activating at least a laser source whilemaintaining spacing between the protrusion and the medium; calculating aresistance response, R₀, as a resistance of the thermal sensor with nobias power supplied to the thermal sensor; in the absence of laserexcitation, determining a first resistance response of the thermalsensor to varying clearance while maintaining spacing between the sliderand the medium; calculating a first value indicative of R₀ dependence onclearance based on the first resistance response; in the presence oflaser excitation, determining a second resistance response of thethermal sensor to varying clearance while maintaining spacing betweenthe slider and the medium, wherein the laser excitation causesprotrusion of a portion of the ABS; calculating a second valueindicative of R₀ dependence on clearance based on the second resistanceresponse; and determining a magnitude of at least a portion of theprotrusion using the first value and the second value.
 16. The method ofclaim 15, wherein: the protruded portion of the ABS comprises a firstregion and a second region; the second region is closer to the mediumthan the first region; and determining the magnitude of at least theportion of the protrusion comprises determining the magnitude of thefirst region.
 17. The method of claim 15, wherein: the protruded portionof the ABS comprises a first region having a first thermal time constantand a second region having a second thermal time constant; the secondregion is closer to the medium than the first region; and determiningthe magnitude of at least the portion of the protrusion comprisesdetermining the magnitude of the first region.
 18. The method of claim15, wherein the portion of the ABS comprises a writer and a near-fieldtransducer of the slider.
 19. The method of claim 15, wherein theportion of the ABS comprises a reader of the slider.
 20. The method ofclaim 15, further comprising determining the magnitude of at least theportion of the protrusion at different diameters or zones of the medium.